This invention relates in general to acoustic devices and in particular to an acoustic wave device having a piezoelectric substrate that utilizes a monolithic antenna to excite acoustic waves in the substrate.
Piezoelectric materials, such as crystalline quartz, generate an electric field or voltage when subjected to mechanical stress, and conversely, generate mechanical stress when subjected to an electric field or voltage. Accordingly, piezoelectric materials have proven useful in many diverse technologies. Typically, electrodes are deposited upon the surface of the crystal and an AC voltage is applied to the electrodes to generate an electric field in the crystal. The electric field, in turn, generates mechanical stresses in the crystal. If the applied AC voltage is at or near the resonant frequency of the crystal, or overtone harmonics of the resonant frequency, resonant acoustic waves are excited within the crystal. The resonant frequencies are determined by the cut angle, thickness, length, width, and mass of the crystal, and the resonant acoustic waves propagate and resonate within the crystal with very little loss.
A measure of how narrow a band of frequencies can be passed through a particular piezoelectric crystal with minimum attenuation relative to the resonant frequency of the crystal is referred to as the Q of the crystal. The Q of the crystal, which is a function of the crystallographic orientation of the crystal, determines the specific application for the crystal. For example, very low Q crystals are capable of converting wide frequency bands of mechanical energy to electrical energy; and, conversely, wide frequency bands of electrical energy to mechanical energy. Thus, low Q materials are often used as sonic transducers in microphones or speakers because the low Q allows many tones to be produced. With a very high Q material, only a very narrow band of frequencies may be passed through the crystal. Thus, high Q material is typically used in devices that require highly accurate frequency control, such as oscillators.
High Q piezoelectric materials also are used in sensors. With modern manufacturing methods, precision crystals of quartz or other similar very high Q material may be made to oscillate at a frequency that is accurate to within a few parts per million or less. During production of such quartz resonators, layers of conductive electrode material may be deposited with a precision of a few atomic layers. The resonant frequency of the resulting resonators will be sensitive to extremely small changes in the mass of the electrodes. This characteristic sensitivity of high Q piezoelectric materials to changes in mass has led to a number of diverse sensor applications. For example, a quartz resonator may be coated with a sorbent that is selective to a particular compound. The amount, or concentration, of the compound can then be determined by monitoring the change in the resonant frequency of the quartz crystal as the compound is absorbed by the sorbent since, as the compound is absorbed, the mass of the sorbent and, hence, the total mass of the vibrating structure increases. Because the addition or subtraction of mass to the piezoelectric material results in a change of the resonant frequency of the crystal, such devices are commonly referred to as a Quartz Crystal Microbalances (QCM's) and are widely used in applications where a change in mass, density or viscosity is monitored, such as in sensing applications.
Referring now to the drawings, a typical known QCM sensor is illustrated generally at 10 in FIGS. 1 and 2. The sensor 10 includes a disc shaped substrate 12 of quartz having a diameter of approximately 25 mm. The standard crystallographic orientation used is an AT-cut since it is a temperature stable orientation in which only a Transverse Shear Mode (TSM) acoustic wave can be excited. Other orientations in quartz in which only a TSM acoustic wave can be excited also may be utilized. FIG. 1 shows the reference surface 14 of the substrate while FIG. 2 shows the sensing surface 16 of the substrate 12 that is opposite from the reference surface 14. A disc shaped reference electrode 18 formed from an electrically conducting material and having a diameter of approximately 6 mm is deposited upon the center of the reference surface 14. The electrode 18 is formed from an electrically conductive metal. The reference electrode 18 is connected by a first strip 20 of conductive material to an arcuate reference electrode tap 22. The reference electrode tap 22 allows electrical connection to an external sensing circuitry (not shown). The electrical connection is illustrated by a wire lead 24; however, the lead 24 is intended to be exemplary and other types of conventional electrical connections may be utilized.
As shown in FIG. 2, a disc shaped sensing electrode 26 formed from an electrically conductive metal and having a diameter of approximately 13 mm is deposited upon the center of the sensing surface 16. A second strip of conductive material 28 extends from the sensing electrode 26 to the edge of the sensing surface 16, transversely across the side of the substrate 12 and onto the reference surface 14, as shown in FIG. 1, where it ends in an arcuate sensing electrode tap 30. Similar to the reference electrode tap 22, the sensing electrode tap 30 allows electrical connection to the external sensing circuitry (not shown), as illustrated by a wire lead 32. Additionally, an adhesive layers 33 and 34 are typically deposited between the electrodes, 18 and 26, the corresponding substrate surface, 14 and 16, respectively, to enhance adherence of the electrodes to the substrate surface. Finally, depending upon the application, a sorbent selective film (not shown) may cover the sensing surface 16.
During operation of the sensor 10, a variable frequency oscillator (not shown) is electrically connected to the reference and sensing electrode taps, 22 and 30, and the sensing surface 16 is inserted into an environment, which may be either a gas or a liquid, while the reference surface 14 remains exposed to air. The environment contains a measurand, which is a specific property of the environment that is being sensed by the sensor, such as, for example the concentration of a certain substance within a gas or liquid. Thus, when the sensing surface 16 is inserted into an environment, the sensing surface is exposed to a specific measurand contained within the environment. Should the sensing surface be covered by a sorbent film, the sorbent film also is immersed in the environment. The oscillator applies a varying voltage to the electrodes, 18 and 26, which then generate acoustic waves within the substrate 12. Such a mode of operation is referred to as Thickness Field Excitation (TFE). Before exposing the sensing surface 16 to the measurand the sensor 10 is calibrated by varying the oscillator frequency to resonate the sensor 10. The resonance frequency is detected and stored in a conventional device or circuit (not shown). After calibration, the sensing surface is inserted into the environment being monitored. The effect of mechanical loading properties of the measurand, such as mass, density and viscoelasticity, upon the sensing surface 16 causes the resonant frequency of the sensor to shift. The shift in resonant frequency can be calibrated to be indicative of the magnitude of a specific mechanical loading property of the measurand.
Alternate embodiments of the QCM sensor 10 having different sensing electrodes are illustrated in FIGS. 3 through 5. FIG. 3 illustrates small electrode geometry with a very small circular sensing electrode 35. A typical diameter for the sensing electrode 35 would be about 0.8 mm. In FIG. 4, a closed ring geometry sensing electrode 36 that has an aperture formed through the center of the electrode disc is shown, while FIG. 5 illustrates an open ring sensing electrode 38. The open ring electrode 38 is very similar to the closed ring electrode 36, except that the open ring electrode 38 has a slot 40 extending through the ring that corresponds to the tap region of the reference electrode. Both the closed and open ring electrodes 36 and 38 have an outside diameter of approximately 13 mm and an inside diameter of approximately 11 mm. All of the sensors shown in FIGS. 4 through 5 have a reference surface configuration that is similar to the sensor 10 shown in FIG. 1.
The use of conventional QCM sensors, such as the one shown in FIGS. 1 and 2, is limited to applications only the mechanical properties listed above are measured. In addition, the resonant frequency of the device is limited to the fundamental frequency of the device, which limits the sensitivity of the device. In many applications, the measurement of changes in the electrical properties is critical. However, with conventional QCM sensors, such as the one shown in FIGS. 1 and 2, the sensing electrode 26 that contacts the measurand is the same size or larger than the reference electrode 18 that contacts air. Because of its size, the sensing electrode 26 shields most of the TSM electric field, preventing the penetration of the field into the measurand. Thus, a conventional QCM sensor has minimal sensitivity to changes in electrical properties of the measurand. The modified sensing electrode geometries shown in FIGS. 3 through 5 reduce the size of the sensing electrode. As a result, a small shift of the resonant frequency of the modified QCM sensors may be detected as the electrical properties of the measurand changes.
In order to allow Transverse Shear Mode (TSM) electrical fields to penetrate, a sensing surface of a AT-cut quartz substrate that is exposed to liquid or gas should be bare. Such a bare sensing surface can be achieved by placing both electrodes upon a reference surface, that is opposite from the sensing surface, to provide a Lateral Field Excited (LFE) sensor. The details regarding such a LFE sensor and application of this sensor to detect phosmet and E. coli are described in U.S. Pat. No. 7,075,216, which is incorporated herein by reference.
In contrast to QCM and LFE sensors, it is also known to utilize a spiral coil as the excitation source to form two other acoustic wave sensors, namely a Magnetic Acoustic Resonant Sensor (MARS) 41, as shown in FIG. 6, and an Electromagnetic Piezoelectric Acoustic Transduction Sensor (EMPAS) 42, as shown in FIG. 7.
The MARS 41 utilizes the same basic configuration and operating principles as an Electromagnetic Acoustic Transducers (EMAT), a technology that has been used for more than 50 years on the macro scale to test the structural integrity of metallic objects such as sheet metal and materials characterization, but applies it on the micro scale to excite an acoustic wave. In the configuration of the MARS 41 shown in FIG. 6, an electrically excited hand wound spiral coil 43 is placed near, but spatially separated from, a non-piezoelectric substrate 44 that carries a metalized conductive coating, or metal layer, 45 disposed upon the surface of the substrate 44 that is adjacent to the coil 43. Thus, an air gap separates the coil 43 from the surface of the metal layer 45. The substrate 44 is exposed to a permanent magnetic field generated by an adjacent permanent magnet 46. The wound spiral coil 43 produces electromagnetic fields that induce eddy currents on the thin metal layer 45 that is attached to the substrate 44. The permanent magnet 46 produces static magnetic fields that couple with the time-varying eddy currents to produce time varying Lorentz forces within the metal layer 45. These time varying Lorenz forces produce time-time varying stresses and hence acoustic waves within the substrate 44. As with other acoustic wave sensors the resonant frequency of the MARS 41 shifts with changes on its sensing surface. Unlike other acoustic wave sensors however, the MARS configuration has the advantage of utilizing non-piezoelectric substrates such as aluminum, silica glass, sapphire and high-Q silicon membranes.
The EMPAS 42 shown in FIG. 7 utilizes a piezoelectric crystal as a substrate 47 and a hand wound spiral coil 43 that is separated by a small air gap of approximately 30 mm from the substrate. A thin plastic o-ring (not shown) is placed between the coil 43 and the substrate 47, resulting in the small air gap between the coil and crystal. The spiral coil 43 produces electric fields that penetrate the piezoelectric material to excite acoustic waves within the substrate 47. The EMPAS 42 has been shown to operate at frequencies up to 700 MHz.
In both the MARS 41 and EMPAS 42, the sensor configurations are not monolithic and contain several components that may result in poor reproducibility of sensor properties from sensor to sensor. Furthermore, in both the MARS and EMPAS sensors, the spiral coils are hand wound and are separated from the substrate by an insulating layer of air. Although these sensors have been shown to operate at frequencies of approximately 700 MHz, reproducibility of sensor properties from device to device is problematic. Because increased accuracy of sensors is desirable, it would be desirable to devise a device having an improved geometry.